A basic layer 2 Ethernet switch will flood multicast traffic out of all its ports, because it is unaware of which device might want to pick up a stream. Devices on those ports will ignore the frames (packets) if not receiving the streams, but they will take up bandwidth on the switch port, wire and network card.
To avoid this problem, many Ethernet switches support IGMP Snooping. When you turn this feature on, the switch stops sending out any multicast frames (packets) and it listens to the IGMP Join reports coming from devices on its ports (which are being sent to the Router, hence ‘snooping’). When it picks one up, it then knows that a device on a particular port wants to receive a stream and it will start sending out of that port only.
See IGMP, Multicast
